Field of the Disclosure
The technology of the present disclosure relates to distributed antenna systems that are capable of distributing wireless radio-frequency (RF) communications services over wired communications mediums.
Technical Background
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, “wireless fidelity” or “WiFi” systems and wireless local area networks (WLANs) are being deployed in many different types of areas (coffee shops, airports, libraries, etc.). Wireless communication systems communicate with wireless devices called “clients,” which must reside within the wireless range or “cell coverage area” in order to communicate with the access point device.
One approach to deploying a wireless communication system involves the use of “picocells.” Picocells are radio frequency (RF) coverage areas having a radius in the range from about a few meters up to about twenty (20) meters. Picocells can be provided to provide a number of different services (e.g., WLAN, voice, radio frequency identification (RFID) tracking, temperature and/or light control, etc.). Because a picocell covers a small area, there are typically only a few users (clients) per picocell. Picocells also allow for selective wireless coverage in small regions that otherwise would have poor signal strength when covered by larger cells created by conventional base stations.
In conventional wireless systems, as illustrated in FIG. 1, picocell coverage areas 10 in a distributed communications system 12 are created by and centered on remote antenna units 14 connected to a head-end equipment 16 (e.g., a head-end controller or head-end unit). The remote antenna units 14 receive wireless communications services from the head-end equipment 16 over a communications medium 17 to be distributed in their coverage area 10. The remote antenna unit includes information processing electronics, an RF transmitter/receiver, and an antenna 18 operably connected to the RF transmitter/receiver to wireless distribute the wireless communication services to wireless client devices 20 within the coverage area 10. The size of a coverage area 10 is determined by the amount of RF power transmitted by the remote antenna unit 14, receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the client device 20. Client devices 20 usually have a fixed RF receiver sensitivity, so that the properties of the remote antenna unit 14 mainly determine the picocell coverage area size.
One problem that can exist with wireless communication systems, including the system 10 in FIG. 1, is the multi-path (fading) nature of signal propagation. This simply means that local maxima and minima of desired signals can exist over a picocell coverage area. A receiver antenna located at a maximum location will have better performance or signal-to-noise ratio (SNR) than a receiver antenna located in a minimum position. Signal processing techniques can be employed to improve the SNR of wireless data transmission in such wireless communication systems. For example, spatial diversity can be utilized in instances involving many access points. Other signal processing techniques include Multiple Input/Multiple Output (MIMO) techniques for increasing bit rates or beam forming for SNR, or wireless distance improvement. MIMO is the use of multiple antennas at both a transmitter and receiver to increase data throughput and link range without additional bandwidth or increased transmit power. MIMO technology can be employed in distributed antenna systems (DAS) to increase the bandwidth up to twice the nominal bandwidth.
Even with the potential doubling of bandwidth in a distributed communication system employing MIMO technology, a client device must still be within range of two MIMO antennas to realize the full benefits of increased bandwidth of MIMO technology. Ensuring uniform MIMO coverage may be particularly important for newer cellular standards, such as Long Term Evolution (LTE), where increased bandwidth requirements are expected by users of client devices in all coverage areas.
Current MIMO distributed communication systems may not provide uniform coverage areas, particularly in the edges of coverage cells. In this regard to further illustrate this problem, FIG. 2A illustrates a portion of exemplary MIMO coverage areas 10 in the distributed communications system 12 in FIG. 1. The MIMO coverage areas 10 in FIG. 2A are provided by two remote antenna units 14(1), 14(2), which are separated at a distance D1 from each other. Each remote antenna unit 14(1), 14(2) has two antennas 18(1)(1), 18(1)(2) and 18(2)(1), 18(2)(2) respectively. The antenna pairs 18(1)(1), 18(1)(2) and 18(2)(1), 18(2)(2) are each capable of being configured to be intra-cell bonded together to operate in MIMO configuration. By intra-cell remote unit antenna bonding, it is meant that an antenna pair 18 in a particular remote antenna unit 14 are both involved in communications with a particular client device to provide MIMO communications. The first remote antenna unit 14(1) provides a first MIMO coverage area 22(1) using antennas 18(1)(1) and 18(1)(2). The second remote antenna unit 14(2) provides a second MIMO coverage area 22(2) using antennas 18(2)(1) and 18(2)(2). A wireless client device (not shown) located within the first MIMO coverage area 22(1) will receive MIMO services by remote antenna unit 14(1), because the client device will be in range of both antennas 18(1)(1) and 18(1)(2). Similarly, a client device located within the second MIMO coverage area 22(2) will receive MIMO services by remote antenna unit 14(2), because the client device will be in range of both antennas 18(2)(1) and 18(2)(2).
If a client device is located in a coverage area 24 outside or on the edge of the first and second MIMO coverage areas 22(1), 22(2), the client device may still be in communication range of at least one of the antennas 18 of the remote antenna units 14(1), 14(2) to receive communications services. However, the client device will not be in communication range with sufficient SNR ratio of both antenna pairs 18(1)(1), 18(1)(2) or 18(2)(1), 18(2)(2) of a remote antenna unit 14(1), 14(2), and thus will not receive MIMO communications services. FIG. 2B illustrates an exemplary graph 26 illustrating one relationship between antenna 18 separation of the remote antenna units 14(1), 14(2) and MIMO condition number (CN) in decibels (dB). For a 700 MHz communications service frequency, the allowed maximum antenna 18 separation is approximately twenty (20) meters for MIMO capacity of six (6) bits per section per Hertz (s/Hz), assuming a condition number of 60 dB illustrated as line 28. At a 2.6 GHz communications service frequency, the allowed maximum antenna 18 separation is approximately ten (10) meters for MIMO capacity of six (6) bits per section per Hertz (s/Hz), assuming a condition number of 60 dB.
An increased number of remote antenna units could be provided to reduce the maximum separations between MIMO antennas, and thus reduce or eliminate non-MIMO coverage areas. However, providing an increased number of remote antenna units in a distributed communications system increases system cost. Also, providing an increased number of remote antenna units can add additional complexity and associated cost by requiring support of a greater number of remote antenna units in the distributed communications systems.